Understanding defects, testing and inspection - Concrete Repair - page 24

hot, dry climates, but can easily occur on hot days in flat slabs especially
where inadequate attention to protection and curing has been given.
Thermal cracking and delayed ettringite formation
Thermal cracking of concrete can occur in large pours. Typically concrete
can gain in temperature about 14°C per 100 kg of cement in a cubic metre
of concrete. In large pours this sets up a thermal gradient, with the outer part
of the concrete cooling more rapidly than the core. This puts the outer skin
in tension, and small cracks form. With the addition of subsequent drying
shrinkage, the cracks can become quite large ( Figure 1.17).
During the hydration process, it is quite normal for ettringite (calcium
sulfoaluminate) to form inside the concrete. This mineral is normally
associated with sulfate attack, but in the context of a setting concrete is
quite normal. Any expansion resulting from its formation is taken up in the
still-plastic concrete. However, if the temperature of the concrete exceeds
about 70°C then formation of the ettringite can be delayed until after the
concrete has hardened. In this situation, if a source of moisture is present,
the concrete can suffer from quite severe expansion and cracking. Cracking
due to DEF (Figures 1.18 and 1.19) can be mistaken for that due to ASR
and is similar in nature (but not cause). There is a correlation with the alkali
content of the cement, too. The higher the alkali content, the lower the
temperature at which DEF can occur (Grantham et al., 1999).
Figure 1.17 Thermal cracks in the wall of a water tank.